Map-Scale Structures and Contact...

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Geol341-Structural Geology Map - Scale Structures and Contact Relationships

Transcript of Map-Scale Structures and Contact...

Geol341-Structural Geology

Map-Scale Structures

and Contact

Relationships

Last Lecture

• The downward pull of subducting slabs drives plate motions

• All plate motions are rotations

• 3 kinds of plate boundaries create 3 different structural environments

• Gravity causes deformation at passive margins

• Continents grow by accretion at their edges

What type of plate boundary is

there along the US East coast?

Passive Margin

Continental Growth

• Material plastered onto the continents at

convergent boundaries

• Example:

– Everything west of Nevada has been added to

North America in last 700 Ma

Continental Growth

Today’s Outline

• Stratigraphic contacts

– Unconformities

• Primary Sedimentary Structures

– Younging direction

• Intrusive contacts

– Sills and dikes

– Batholiths

• Fault Contacts

– Normal, thrust, strike-slip

• Folds

– Anticlines, synclines

1. Superposition: younger layers on top

Red layers deposited over

tan

Tan sediment deposited

over older rock

Third layer is

youngest and is

on top

Young

Old

Stratigraphic Succession

Basalt Flows- Snake River Plain

Horizontality of Sedimentary

Successions

North Slope, AK

Horizontality of Sedimentary

Successions

North Slope, AK

Without vertical exaggeration

Basic Question:

Is the section conformable?

• Is the sedimentary pile continuous?

Unconformities

Disconformity

Angular Unconformity

Angular unconformity

Death Valley

The Great Unconformity

Grand Canyon, AZ

1.1-1.2 Ga

1.7 Ga

0.5 Ga

Angular Unconformity

In seismic data

Onlap Surface

North Slope, AK

Fossils & disconformities

Correlation = determine equivalent units

I

H

G

F

E

D

C

B

A

I

H

C

B

A

Is the sedimentary succession

right-side up?

• How can

we tell?

Which is older. A or B?

AB

Is the sedimentary succession

right-side up?

• How can

we tell?

Primary Structures

Pillow Basalts Up

Sedimentary StructuresStromatolites, Belt Gp., MT

Up

Ripples

Cross bedded sandstones

Graded beds

Up

Intrusive Contacts

They cut older rocks

• Dikes

• Sills

• Stocks

• Batholiths

Coarse-Grained

Fine-Grained

Intrusive Structures

Dikes

Sills

Stock or

batholith

Shiprock, New Mexico

Stock

Dike

Sierra

Nevada

Batholith

Yosemite N.P.

Cross Cutting RelationshipsYounger intrusive cuts through older folds

Amato photo, Burro Mts, NM

1.6 Ga Amphibolite

1.4 Ga

Dikes

Geologic history of an area

based on contact relationships

Orientation of a Plane

Salisbury Crags, Edinburgh, UK,

BGS photo

Jointed gabbro sill

Strike and dip on a map

North

34

38

28

30

Blue Fm.

Grey Fm.

Angular

Unconformity

Map View

Alabama

uK1 =Upper

Cretaceous

IP1 =Permian

20

22

60

Visualizing

the 3D

Structure6

Cross Section

W E

Folds

• Syncline

– (concave up)

• Anticline

– (concave down)

Domes and Basins

Trend and Plunge

• Used for linear features

• Trend = Azimuth of a line

• Plunge = Inclination of a line

Plunging Fold

Plunging

Folds

Does the arrow point at a syncline or anticline?

A. Syncline

B. Anticline

C. Neither

Sheep Mountain Anticline, WY

Fault Contacts

• Normal

• Thrust, reverse

• Strike-slip

Thrust Fault

Faults in Map View

Teeth Point down dip

Pen

Trias

Rose Diagram

Bins # Measurements

0-9 0

10-19 2

20-29 5

100-109 3

Rose Diagram Examples

Software

• Stereonet (free)

• Openstereo (free)

• StereoNet (free)

• Visible Geology (on line app- watch the

tutorial)

• Lambert (iphone app)

• Rockworks (commercial)

How the compass works

Magnetic Declination

True

North

Magnetic

North

USGS Topographic Maps

MN- Magnetic North

- True North

GN- Geomagnetic North

Morgantown Quadrangle

http://www.ngdc.noaa.gov/geomag-web/#declination

Morgantown Declination

9o West

Adjusting Your Compass

Front Back

Key concepts• Stratigraphic contacts

– Unconformities

• Primary Sedimentary Structures– Younging direction, right-side-up?

• Intrusive contacts– Sills and dikes

– Batholiths

– Cross cutting relationships

• Interpreting sequence of events

• Folds– Anticlines, synclines

• Fault Contacts– Normal, thrust, strike-slip